This invention relates to the measurement of magnetic fields produced by the brain of a human subject, and, more particularly, to an approach for performing such measurements using an array of sensors surrounding the head of the subject.
The human brain produces electrical signals. These electrical signals are very faint, but they can be measured noninvasively by various approaches. One such technique, biomagnetometry, is based upon the measurement of the magnetic fields produced outside the head of the subject by the electrical current flows of the brain.
A biomagnetometer is a specially adapted, highly sensitive device having a magnetic field sensor, a detector of electrical current flow in the sensor, and associated electronics. The magnetic field sensor is typically a single-loop or multiple-loop coil of wire which produces a small current flow when a magnetic flux penetrates the loop. The sensor is desirably placed as closely as possible to the head of the subject whose brain signals are to be measured, because the strength of the magnetic field decreases rapidly with distance from the source. The detector is typically a superconducting quantum interference device ("SQUID"), which can detect very small electrical currents.
The sensor and the detector are made of superconducting materials. They are operated at very low temperatures in order to attain their superconducting states and also to suppress noise sources that increase with increasing temperature. Currently available sensor/detector elements are operated at liquid helium temperature, about 4.2.degree. K. In order to be maintained at this temperature, the sensor and detector are placed into an insulating vessel termed a dewar, and cooled with liquid helium. A typical dewar is about 24 inches in diameter and 48 inches in length. The size of the dewar and the need to place the sensors as closely as possible to the head of the subject dictate careful geometric design of the dewar. In the usual practice, the sensors are placed into a small-diameter extension of the main dewar vessel, termed a dewar tail, that can be positioned closely to the head of the subject.
The preceding discussion has described a single measurement channel having a single sensor and its associated single detector. The earliest biomagnetometers were built around a single measurement channel, but later designs have incorporated multiple measurement channels into a single unit. Current biomagnetometers have tens of measurement channels, and future instruments may have even more.
An important trend in the advance of biomagnetometry is the development of a capability for full-head coverage of subjects. That is, the sensors may be arranged in an array that is positioned around the head of the subject. The magnetic fields produced by the brain of the subject are measured by all of the sensors simultaneously. The measurements are analyzed to determine the position and strength of the source or sources within the brain.
Various methods for positioning, cooling, and supporting the full-head array of sensors have been proposed. In one, the lower end of the dewar is shaped in the manner of a helmet that fits over a portion of the entire head of the patient. The subject sits fully upright in a chair, and the dewar is lowered over the head of the subject until as close a fit as possible is attained. The sensors are immersed in a liquid helium reservoir inside the dewar and positioned about the inner surface of the helmet-shaped recess. By this approach, the well-known technology of existing biomagnetometers is used with a specialized configuration of the lower end of the dewar in order to perform full-head measurements of the subject.
The present inventor has recognized that, while such an approach is operable and useful, it also has shortcomings. Perhaps most importantly, the presently proposed biomagnetometers having full-head coverage are operable only when the subject is sitting in a rigidly defined upright position. Many subjects cannot be presented in an upright position due to their illnesses or infirmities. In one important application, the subject may be a candidate for surgery that is to be performed with the patient in a reclining position. The biomagnetic measurements are used to aid the neurosurgeon in planning the surgery, which must be conducted very precisely in order that vital areas of the brain not be damaged. A measurement performed on the sitting subject may not be applicable for certain surgical procedures undertaken with the patient reclined because of a slight shifting of the location of the brain that is known to occur between the sitting and the reclining positions. Thus, it is highly desirable to perform the biomagnetic measurements with the subject in the reclining position for this particular application.
The dewar containing the sensors cannot be arbitrarily positioned at any desired angle in order to fit the helmet-shaped recess over the head of the subject, because the dewar contains liquefied gas that can shift to expose otherwise-submerged detectors or even spill if the dewar is tilted at too steep an angle. A high tilt angle of the dewar can also expose and effectively short thermal pathways, resulting in a high evaporation rate of the liquefied gas. One solution to the problem of performing full-head measurements is to supply two dewars, one for sitting and one reclining subjects. This approach is expensive and not fully satisfactory, because it may be desirable to position the subject at an intermediate position between upright and reclining positions. Various types of dewars with movable lower ends can also be envisioned, but these designs are complex, are subject to leaks, are heavy, and are not readily realizable with currently available materials.
There is a need for an improved approach to performing full-head biomagnetic measurements of a subject. The present invention fulfills this need, and further provides related advantages.